A perpendicular magnetic recording system is drawing attention as a technology for achieving high density in magnetic recording, to replace a conventional longitudinal magnetic recording system. A double layer perpendicular magnetic recording medium, in particular, is known as a perpendicular magnetic recording medium suited for providing high density recording. See for instance, Japanese Patent Publication No. S58-91. The double layer perpendicular magnetic recording medium is provided with a soft magnetic film called a soft magnetic backing layer under a magnetic recording layer, which serves to record information. The backing layer facilitates to pass the magnetic flux generated by a magnetic head and has high saturation magnetic flux density Bs. The double layer perpendicular magnetic recording medium enhances magnetic field intensity and magnetic field gradient generated by a magnetic head, improves resolution of records, and increases leakage flux from the recording medium.
The soft magnetic backing layer is generally composed of an alloy film of Ni—Fe, or Fe—Si—Al, or an amorphous alloy film of mainly cobalt, formed by a sputtering method and has a thickness ranging 200 nm to 500 nm. However, forming such a relatively thick film by a sputtering method is unfavorable from the viewpoint of production cost and mass productivity.
To solve this problem, a soft magnetic backing layer formed of a soft magnetic film by an electroless plating method has been proposed. Feasibility of a soft magnetic backing layer is suggested with the proposed materials of Co—B film in Japanese Unexamined Patent Application Publication No. H5-1384 and Ni—Fe—P film in Japanese Unexamined Patent Application Publication No. H7-66034.
In a conventional magnetic recording medium of a longitudinal magnetic recording system that is used as a hard disk drive, a nonmagnetic substrate is provided with a nonmagnetic Ni—P plating film that contains phosphorus in a concentration of around 11 wt %, with a thickness of 8 μm to 15 μm formed on an aluminum alloy base by an electroless plating method. The nonmagnetic Ni—P plating film mainly serves to fill in the defects like depressions on the aluminum alloy base, as well as to obtain a smooth surface by polishing the surface of the plating film. Further, the nonmagnetic Ni—P plating film is used to secure hardness of the surface required for a substrate of a hard disk. A certain degree of surface hardness is required to prevent the substrate from damage in an event of collision of the magnetic recording medium with the magnetic head during operation of a hard disk drive.
Since the nonmagnetic Ni—P plating film can be made ferromagnetic by heating at a temperature of about 300° C. or higher, a proposal has been made to use the Ni—P plating film as a soft magnetic backing layer of a perpendicular magnetic recording medium. It has been proposed in Japanese Unexamined Patent Application Publication No. H1-285022 to form a soft magnetic Ni—P film by heat treating a nonmagnetic Ni—P plating film at a temperature of 300° C. or higher for use as a soft magnetic backing layer. It has been also proposed in Japanese Unexamined Patent Application Publication No. H10-228620 that by laminating a soft magnetic Ni—P film that is obtained by heat treating a nonmagnetic Ni—P plating film at a temperature between 250° C. and 500° C. and a Sendust film that is formed by sputtering, the Ni—P film helps the Sendust film exhibit its own function, to attain an effective soft magnetic backing layer.
The nonmagnetic Ni—P plating film has already been practically applied to a nonmagnetic substrate of a hard disk as described above. Thus, mass production methods and the surface smoothing technique by polishing are well known. Accordingly, the Ni—P plating film is very promising from the viewpoint of manufacturing cost if the plating film could be transformed to a soft magnetic backing layer by a heat treatment and could be considered for a substrate of a perpendicular magnetic recording medium.
To use the previously described Co—B plating film or Ni—Fe—P plating film for a soft magnetic backing layer, the surface needs to be polished smooth. Because the hardness and workability of these materials are foreseen to be substantially different from those of the nonmagnetic Ni—P plating film, the conventional processing technique for the nonmagnetic Ni—P plating film cannot be applied to process those materials.
For the materials, such as Ni—Fe, Co—Fe, or other alloys of two or more metal components, it is very difficult to control, for example, the composition of the plating bath in an electroless plating method, and thus the quality of such a material is difficult to control and maintain during the mass scale production.
The inventors of the present invention made extensive studies on the transformation of the nonmagnetic Ni—P plating film to a soft magnetic state by a heat treatment and found that the Ni—P plating film cannot be sufficiently transformed to soft magnetic state by heat treating at a temperature of 300° C. or less, and that heat treating at a temperature higher than 300° C., which is necessary to attain soft magnetic state, increases the surface roughness of the plating film. While a commonly used nonmagnetic Ni—P plating film has a homogeneous amorphous structure, heat treating the same to transform it to a soft magnetic state causes formation of both types of crystals of metallic Ni and intermetallic compound of Ni3P. This structural change can be the reason for the increase in the surface roughness. The increase in the surface roughness increases the magnetic head's flying height, which needs to be at a low height to enable high density recording of a hard disk. Consequently, the plating film that is transformed to soft magnetic by such a method can be hardly utilized for a soft magnetic backing layer of a perpendicular magnetic recording medium.
While studies to reduce the surface roughness were made by polishing a Ni—P plating film after the heat treatment, a smooth surface was hardly obtained by polishing the film that was crystallized by the heat treatment. This is because the crystal of metallic nickel and the crystal of intermetallic compound Ni3P have different hardness and exhibit very different workability.
As described above, conventional technologies have been difficult to provide a backing layer of a perpendicular magnetic recording layer that allows high density recording with a low production cost and mass productivity. In addition, a soft magnetic plating film of a substrate for a perpendicular magnetic recording medium has to be designed to have the values of a surface roughness and a surface hardness that are appropriate to use for a substrate.
Accordingly, there remains a need for a technology to provide an effective backing layer for a perpendicular magnetic recording layer that allows high density recording. The present invention addresses this need.